Generally, the high dielectric permittivity of biological media makes it possible to design passive sensors, notably surface wave sensors, operating at reduced electromagnetic wavelengths and thereby permitting likewise reduced antenna dimensions.
Solutions have already been proposed, notably described in the patent application BF Goodrich WO 00/56210, in which a passive surface acoustic wave (commonly dubbed “SAW”) pressure sensor attached to a stent makes it possible in particular to measure blood pressure in animals or humans.
In the case considered, the mode of interrogation between the transmitter/receiver situated outside the living being and the implanted sensor is performed by inductive coupling. This mode of interrogation commonly used within the realm of sensors on passive silicon uses frequencies of possibly as high as the 13.56 MHz ISM band. Beyond these frequencies the losses: magnetic losses, eddy current losses (which increase with the square of the frequency), Joule effect losses become very significant and limit the interrogation distances to values which are no longer of interest in relation to the applications concerned.
It is however not possible to use the ISM band at 13.56 MHz to interrogate this type of device. Indeed, the size of an SAW resonator at this frequency ought to be of the order of 50 mm by 10 mm, thus ruling out implantation in the artery of a human being in particular.